Encapsulated coils and method of making same



Sept. 7, 1954 H. E MALONE ETAL 2,688,569

ENCAPSULATED COILS AND METHOD OF MAKING SAME Filed Aug. 2l 1951 )frm/wir Patented Sept. 7, 1954 2,688,569 ENCAPSULATED colLs AND METHOD or AME MAKIN Herbert E. Malone, Riverside, Ill., and Franklin B. Rinck, Indianapolis, Ind., assignors to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application August 21, 1951, Serial No. 242,918 Claims. (Cl. 117-75) This invention relates to encapsulated articles and methods of making the same.

This application is a continuation-in-part of our copending application Serial No. 658,154, filed May 8, 1946, now abandoned.

In the manufacture of transformer coils to be used under rapidly changing atmospheric conditions, for example, in planes, the need arises to protect these coils and to so construct them that they will be substantially unaffected electrically by the varying atmospheric conditions with a minimum of added structural weight. Various insulative impregnants and methods of impregnation have been heretofore used but not altogether successfully. In many cases the impregnant would protect the coil against atmospheric changes but, because of its chemical composition, it would attack either the insulation or the metallic components of the transformer such as the wire winding, the frame and the shielding. Other impregnants that do not attack the components of the coil, after hardening are full of voids in which the trapped air expands and contracts under varying temperature conditions thereby cracking the coating and exposing the coil to moisture and the deleterious effects resulting therefrom.

Objects of this invention are to provide a new and eflicient encapsulated article and a method of encapsulating articles to efficiently protect them from adverse atmospheric conditions.

In accordance with one embodiment of this invention, a transformer coil is first impregnated with a solvent-containing polymerizable plastic material and then is baked to drive olf the solvents and to fully polymerize the solids in the material. The coil is next impregnated with a solventless polymerizable plastic material and again baked to fully polymerize the solventless resin. The fully impregnated coil is then encapsulated by dipping it in a talc-varnish mixture, the coat being air-dried for a definite period of time and cured by controlled underbaking of the varnish to partially polymerize the varnish to obtain a final flexible coat.

A complete understanding of the invention may be had by reference to the following description taken in conjunction with the accompanying drawings, wherein Fig. l illustrates a transformer impregnated and coated in accordance with the preferred embodiment of this invention; and

Fig. 2 is a cross-section of that portion of Fig. 1 encircled by the dot and dash line, and showing in exaggerated detail a cross-section of the coil conductors and the impregnant filled interstices therebetween.

In the preferred embodiment of this invention, an open type transformer IU, having a coil l l wrapped with tape l2, such as, for example, a fabric tape or one composed of other insulating material, and a core I3, is nrst dried in an evacuated chamber at an elevated temperature of about 220-240 F. for about four hours to remove any moisture that might be contained in the coverings on the conductors of the coil, after which it is removed to another chamber and allowed to cool in a vacuum to room temperature. The conductors of the coil may be composed of stranded copper, and the coverings may be composed of paper, fabric, enamel, rubber or other suitable insulating material. Since the transformer is hot while it is being transferred from one chamber to the other, it will not acquire any moisture during the transfer. The transformer is then impregnated with a solvent-containing polymerizable insulating varnish by admitting the impregnant into the drying chamber until the transformer is submerged.

This first impregnation is carried out in a minimum vacuum of 28 of mercury. After impregnating, the transformer is baked at such a temperature and for such a length of time as to volatilize and drive oif the solvents in the varnish and to fully polymerize the solids in the varnish. Twelve to fourteen hours of baking at 269 F. has been found to be satisfactory. The transformer is baked with one end of the coil down so that the surplus insulating varnish will collect in the bottom of the coil and will permeate the tape l2 to form a retaining cup I4 (Fig. 2) for a later impregnating composition.

The insulating varnish may be Harvel varnish or a heat reactive synthetic alkyd resin type varnish which will not chemically attack the materials in the transformer, or it may be a phenol formaldehyde type which will not chemically attack the materials in the transformer. Harvel varnish is a natural product made by extracting the oils from the cashew nut and is a condensation product of cashew nut shell oil and a formaldehyde reaction product (which contains a phenolic component) dissolved in a suitable solvent, such as, for example, petroleum naptha. One varnish which has been found quite satisfactory for the impregnating compound is Harvel 612C varnish, manufactured by the Irvington Varnish and Insulator Company, which may be used unchanged or with a small amount of as the impregnant. The varnish contains from 40% to 60% solvents, and when fully polymerized, only the solids remaining, there will be a shrinkage of about 50%, resulting in shrinkage throughout the transformer. If no further treatment were given to the transformer, the air in the spaces left by the shrinkage would contract and expand in a breathing action when exposed to sudden variations in temperature extremes; Such a breathing action would'cause any outer coating of baked varnish to crack, thereby creating ssures which would admit moisture to the coils when subjected to high humidities or immersed in water.

In Figure 2 of the drawing, which is a crosssectional view of the completely processed transformer, the spaces left by the shrinkage are shown as spaces I5 after they have been nlled with a second impregnation hereinafter described. A coating I6 is left on the insulated conductors. Il by the first impregnation. To fill the voids left after the first impregnation, the transformer is impregnated a second time with a solventless varnish, also in a minimum vacuum of 28" of mercury by placing the transformer in a chamber which is then evacuated and later filled with the impregnant, to effect the second impregnation, after which the transformer is removed and baked to fully polymerize the resin or varnish. In order to prevent the solventless varnish from running out of the coil, the transformer should be impregnated and baked in the same position as that of the first impregnation so-that full advantage may be taken of the cuplike shell i4 formed by the rst impregnation. The solventless varnish may comprise monomers of styrene and an ester of a polybasic acid which may be copolymerized to form the thermosetting resins with an added accelerator such as benzoyl peroxide. Other accelerators which may be used are tertiary butyl hydroperoxide and lauroyl peroxide. Satisfactory examples of the solventless impregnant are varnishes containing (l) a polymerizable unsaturated alkyd resin, more particularly one obtained by partial reaction of ingredients including a polyhydric alcohol and an alpha-unsaturated alpha, beta-polycarboxylic acid, e. g., ethylene glycol fumerate, ethylene glycol maleate, diethylene glycol itaconate, tri-v ethylene glycol maleate, propylene glycol itaconate, glyceryl itaconate, ethylene glycol maleate phthalate, diethylene glycol fumerate sebacate, etc., and (2) a liquid monomer containing an ethylenic linkage that is copolymerizable with such alkyl resin, e. g., styrene, polyallyl (diallyl, triallyl, tetraallyl, etc.) esters of polybasic acids (e. g., inorganic polybasc acids, polycarboxylic acids, etc.), for instance, diallyl oxalate, diallyl malonate, diallyl sebacate, diallyl maleate, diallyl phthalate, diallyl itaconate, triallyl aconitate, triallyl tricarboxylate, triallyl phosphate, etc. The solventless varnish also preferably contains a small amount, for instance, about 0.2 to 5 per cent by weight (of the polymerizable components) of a suitable catalyst, e. g., lauroyl peroxide, stearyl peroxide, ascariodole, benzoyl peroxide, acetyl. peroxide, etc., for accelerating the conversion of the polymerizable component or components thereof to a thermoset or substantially insoluble and infusible state.

In addition to the polymerizable, thermosetting, solventless varnishes hereinbefore mentioned by wayof illustration as being suitable for use as the second impregnant, other varnishes of this class may be employed, examples of which are given in Flynn et al. Patent 2,464,568, issued March 15, 1949, vand in Hill et al. Patent 2,414,525, granted January 21, 1947. Such resinous compositions are commonly known as solvent-less compositions, though in fact a solvent composed of a reacting substance is present. The presence of any iiller material is generally undesirable since the penetration of the resin into the interstices in thek article being impregnated may be hindered. During hardening, the solventless resinous composition should give oi no moisture of condensation or other: gaseous products, since these will tend to cause gas pockets and otherwise deleteriously affect the insulation ofV the member. In other words in preparing the composition, it should be kept in mind that it should be of such a nature that when polymerizing it does not produce moisture or other gaseous products. The composition should harden or polymerize without any significant change in volume over that occupied while in the liquid state.

Baking at a temperature of about 260 F. for about twelve hours has been found to be suitable for full polymerization of the second impregnation, andis conducted after the impregnation. A preferredmethod of treatment comprises placing the coil or coil assembly inY a vacuum chamber where air and other undesirable volatile matter are evacuated from the coil, for instance by holding the chamber under suitably low pressure,- e. g., a pressure of 50 to 2,000 micr0ns,.

for a suitable period, e. g., from 1/2 to 6 hours. Thesolventless varnish is admitted to the chamber in an amount at least suiiicient to cover the coil. The bath of varnish in the chamber is maintained under substantial pressure, e. g., from 50 to 150 pounds per square inch or more, preferably under an atmosphere of a non-explosive gas to which the varnish is inert, e. g.,

nitrogen, argon, helium, krypton, neon, etc., for

a suitable period, e. g., from 1/2 to 4 hours or more. In this way substantially all voids in the coil structure are filled with solventless varnish. The coil is removed from the bath of varnish, freed of excess varnish appearing on the exterior surfaces, e. g., by wiping the coil, allowing the varnish to drain olf, or a combination of both such means. The coil is then treated, for example by placing itin a heated oven, to polymerize the polymerizable, thermosetting, solventless varnish to solid state. It was found that, after polymerization, the shrinkage was not over 3%.. The material added to the transformer as a result ofy the second impregnation is shown at i5 and i8 in Fig. 2, lls all the spaces, and isy kept completely out of contact with coverings 20 insulating.` conductors ll so that it causes no damage to theicoveringwhile providing excellent insulating and weatherproong qualities to the composite impregnations.

In order to provide a flexible and moistureproof, outer coating I9 on the transformer which will have good dielectric qualities, the fully impregnated transformeris then coated all over by dipping it into a mixture of a filler. such as talc and a solvent-type insulating varnish, for ex.-

ample a Harvel varnish of the type disclosed hereinabove, the varnish being air-dried for a definite period and cured by controlled underbaking so that the varnish will not be completely set up and will provide a tough flexible coating. The proportions of the mixture may be three pounds of talc to one gallon of Harvel varnish. A suitable drying period for the coating may be from five to six hours, and baking temperatures under 200 F. have been found satisfactory. For greater protection, a second outer coating of the talc-Harvel varnish mixture may be applied and cured in the same manner as the first coat. The preliminary air-drying step for the coating is necessary to prevent blowholes which may be caused by the surface hardening of the coating before the solvent is completely driven off.

The hereinbefore described process provides a coil which is thoroughly protected against adverse atmospheric conditions. The rst impregnation insulates the transformer components from the second impregnant which might chemically attack the transformer components, but since the rst impregnant contains a high percentage of solvent, the polymerized product after the rst impregnating is shrunk leaving spaces in which air would enter to contract and expand in accordance with ambient temperature changes thereby producing fissures through which moisture might reach vital parts of the transformer if exposed to extreme humidity. The second impregnation serves to fill these spaces thereby driving off any air that might otherwise be entrapped therein and is kept completely out of contact with the elements of the transformer by the first impregnation. The third step, that of coating the entire structure with a talc-Harvel varnish mixture, serves to provide an outer coat that is flexible enough to follow any contraction and expansion of the metal transformer components resulting from temperature variations.

What is claimed is:

1. The method of coating an electrical device including a coil, comprising impregnating said coil with a solvent-type insulating compound chemically inert with respect to said coil, setting said compound, then providing said coil with a tough, impervious, flexible coating by impregnating said coil with a solventless, fluid, unpolymerized, polymerizable compound corrosive to said coil if in direct contact therewith, and polymerizing said solventless coil to form said tough flexible coating.

2. The method of coating an electrical device including a coil, comprising impregnating said coil with a solvent-type, insulating, unpolymerized, polymerizable varnish having cashew nut shell oil, formaldehyde and a volatile solvent as the essential ingredient thereof which is chemically inert with respect to said coil, fully polymerizing said compound, then providing said coil with a tough, impervious, fiexible coating by impregnating said impregnated coil with a solventless, fluid, unpolymerized, polymerizable compound corrosive to said coil if in direct contact therewith, and fully polymerizing said solventless compound to form said tough flexible coating.

3. The method 0f coating an electrical device including a coil, comprising impregnating said coil with a solvent-type insulating resinous material chemically inert with respect to said coil, simultaneously hardening said material and driving off the solvent, then providing said coil with a tough, impervious, iiexible coating by impregnating said coil with a solventless resinous material corrosive to said coil if in direct contact therewith, and hardening the solventless material to form said tough fiexible coating.

4. An article of manufacture comprising an electrical apparatus having a coil, a polymerized coating of solvent-type resinous material impregnating said coil, and a polymerized coating of solventless-type resinous material corrosive to said coil if in direct contact therewith impregnating said rst coating and forming a tough, impervious, flexible coating thereon.

5. An article of manufacture comprising an electrical apparatus having a coil, a fully polymerized coating of solvent-type resinous material impregnating said coil, and a fully polyinerized coating of solventless resnous material corrosive to said coil if in direct contact therewith impregnating the first-said coating and forming a tough, impervious, flexible coating thereon.

References Cited in the le of this patent. UNITED STATES PATENTS Number Name Date 2,414,525 Hill et al Jan. 21, 1947 2,459,018 De Monte et al. Jan. l1, 1949 2,464,568 Flynn et al Mar. 15, 1949 

1. THE METHOD OF COATING AN ELECTRICAL DEVICE INCLUDING A COIL, COMPRISING IMPREGNATING SAID COIL WITH A SOLVENT-TYPE INSULATING COMPOUND CHEMICALLY INERT WITH RESPECT TO SAID COIL, SETTING SAID COMPOUND, THEN PROVIDING SAID COIL WITH A TOUGH, IMPERVIOUS, FLEXIBLE COATING BY IMPREGNATING SAID COIL WITH A SOLVENTLESS, FLUID, UNPOLYMERIZED, POLYMERIZABLE COMPOUND CORROSIVE TO SAID COIL IF IN DIRECT CONTACT THEREWITH, AND POLYMERIZING SAID SOLVENTLESS COIL TO FORM SAID TOUGH FLEXIBLE COATING. 